The most pertinent art known on which this invention is based is U.S. Pat. No. 2,964,604; 12/13/60 to P. C. Jacobs, Jr. et al for CURRENT-LIMITING FUSES HAVING COMPOUND ARC-VOLTAGE GENERATING MEANS. There are numerous other patents following the basic teachings of this patent, but their discussion in this context does not appear to be necessary because the above patent includes all the basic teachings on which subsequent patents are based.
The above patent to Jacobs et al refers to a fuse whose fusible element has a plurality of serially related points of reduced cross-section where arc initiation occurs simultaneously at severe, or major, fault currents, or short-circuit currents. The fusible element is embedded in a pulverulent arc-quenching filler, particularly quartz sand. Some points of reduced cross-section are sandwiched between plates of gas-evolving material such as, for instance, a synthetic-resin-glass-cloth laminate and other points of reduced cross-section are not sandwiched between such plates. The synthetic resin of the plates is preferably melamine. The gas-evolving plates of this kind expose some of the points of reduced cross-section, where arc initiation occurs, to the action of the pulverulent arc-quenching filler, and shield other points of reduced cross-section, where arc initiation occurs, from the action of the arc-quenching filler. Where the points of reduced cross-section of the fusible element are exposed, a relatively high arc voltage is generated. Where the points of reduced cross-section are shielded, the rate of rise of the arc voltage will be less steep than at the points of reduced cross-section where the arc is fully exposed to the action of the arc-quenching filler.
Since the shielding material is gas-evolving, gas evolving from it will play a significant part in the process of arc-extinction. The gas-evolution will increase the pressure in the fuse casing, and increased pressure is helpful in interrupting an arc discharge.
Gas-evolution from a gas-evolving material may also be used to blow an arc in a given direction, where there is less danger of damage by the arc than when the direction of its movement were uncontrolled.
I have also found that fasteners heretofore used to affix a pair of gas-evolving plates to a fusible element can be dispensed with if the gas-evolving plate or the like is sufficiently thick, and the gas-evolving plate or the like is loosely inserted in the gap formed between contiguous fusible elements, or fuse links.